As a projector which projects an image that is displayed on a display element onto a projection surface such as a screen, one that employs a DMD as the display element for its fast switching speed has been in practical use. Recently, movies that can be enjoyed as stereoscopic images (hereinafter, 3D images) have been produced; movie theaters where such movies can be enjoyed have started to be set up, and projection TV sets for household use which are capable of displaying 3D images have started to be commercially available. A projector incorporating a DMD is commonly used as a 3D projector, because it displays images for a right eye and images for a left eye in a time-division manner, and a viewer can enjoy the thus displayed images as 3D images by wearing a pair of polarization glasses or a pair of shutter glasses.
Either pair of glasses mentioned above transmit circularly polarized light or linearly polarized light and direct the light to the right and left eyes of a viewer, and thus, if an image is projected in the form of randomly polarized light by using a projector incorporating a DMD, half or more of the amount of light is lost through the glasses. To prevent this, it is desirable to illuminate the DMD with illumination light having an aligned polarization direction, to obtain a projection image by using polarized light such that light is used efficiently. In this respect, for example, in a projector according to Patent Literature 1, a DMD is illuminated with light generated in the following manner: a polarization conversion optical system performs polarization separation on light from a light source, and then, one light is converted into light (linearly polarized light) having the same polarization direction as the other light.
Note that, in the projector according to Patent Literature 1, for an illumination system to effectively illuminate a light irradiation surface (an image display region) of the DMD, the illumination system including the polarization conversion optical system is disposed such that it is rotated about an optical axis by a predetermined angle (in this case, a polarization separation direction at the polarization conversion optical system corresponds to a long-side direction of the rectangular image display region of the DMD). As a result, linearly polarized light that has an aligned polarization direction and emerges from the polarization conversion optical system is obliquely incident on a reflection surface (for example, a dichroic surface or a reflection surface of a reflection mirror) in an optical path leading to the DMD, that is, the lineally polarized light is incident on the reflection surface such that the polarization direction thereof is in any state but being parallel or perpendicular to an incidence plane. Incidentally, the incidence plane is defined as a plane which, when a central light ray of a light beam travelling toward a center of the image display region of the DMD from the light source is incident on a surface, includes the central light ray which is incident on the surface and a normal line of the surface at the point where the central light ray is incident on the surface.